1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to a method and apparatus for driving an electro-luminescence display device that is capable of preventing a defect of signal lines caused by a relatively high scan voltage and current.
2. Description of the Related Art
Recently, there have been highlighted various flat panel display devices reduced in weight and bulk that is capable of eliminating disadvantages of a cathode ray tube (CRT). Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display, etc.
The EL display device is largely classified into an inorganic EL device and an organic EL device, and has advantages of a fast response speed, high light-emission efficiency, a high brightness and a wide viewing angle. Since the organic EL display device can display a picture at a high brightness of tens thousands of [cd/m2] by a voltage of approximately 10 [V], it has been highlighted as a post-generation display device.
In the organic EL cell, as shown in FIG. 1, an anode 12 is formed from a transparent conductive material on a substrate 11; and a hole injection layer 13, a light-emitting layer 14 made from an organic material and a cathode 15 made from a metal having a low work function are disposed thereon. If an electric field is applied between the anode 12 and the cathode 15, holes within the hole injection layer 13 and electrons within the metal are progressed into the light-emitting layer 14. Then, a phosphorous material within the light-emitting layer 14 is excited and transited to thereby generate a visible light. In this case, the brightness is in proportion to a current between the anode 12 and the cathode 15.
FIG. 2 shows a general passive matrix type EL display device.
Referring to FIG. 2, the EL display device includes a EL display panel 20 having EL cells 18 arranged at intersections between 1st to nth scan lines SL1 to SLn and 1st to mth data lines DL1 to DLm, a scan driver 22 for driving the scan lines SL, and a data driver 24 for driving the data lines DL.
Each of EL cells 18 is selected when a scanning pulse is applied to the scan line SL connected to the cathode to thereby generate a light corresponding to a pixel signal applied to the data line DL connected to the anode, that is, a current signal. Each EL cell 18 can be equivalently expressed as a diode connected between the data line DL and the scan line SL. In each EL cell 18, as shown in FIG. 3, a negative scanning pulse SCAN is applied to the scan line SL and, at the same time, a positive current according to a data signal DATA is applied to the data line DL, thereby radiating it when a forward voltage is loaded. Otherwise, a backward voltage is applied to the EL cells 18 included in the unselected scan lines 18, so that it is not radiated. In other words, forward electric charges are charged into the radiating EL cells 18, whereas backward electric charges are charged into the non-radiating EL cells 18.
The data driver 24 applies a current signal DATA having a current level or a pulse width responding to a data signal for each horizontal period to the 1st to mth data lines DL1 to DLm.
The scan driver 22 applies a negative scanning pulse SCAN to the 1st to nth scan lines SL1 to SLn on a line sequence basis. The scan driver 22 includes first and second switches T1 and T2 connected in a push-pull type as shown in FIG. 4.
The first switch device T1 connected to a scan high voltage source Vhigh supplies a scan high voltage Vhigh to the scan lines SL1 to SLn in response to a control signal A1. While an inverse voltage being applied to the organic EL cells 18 in which the scan high voltage Vhigh is supplied to the cathode 15, a backward current directing from the cathode 15 into the anode 12 is flown into the EL cells 18 to thereby cause a non-radiation of the EL cells 18.
The second switching device T2 connected to a ground voltage source GND sequentially supplies a scan voltage with a ground voltage GND to the scan lines SL1 to SLn in response to a second control signal A2, thereby selecting the scan lines SL1 to SLn at which a data is displayed. A forward current ion directing from the anode 12 into the cathode 15 flows into the organic EL cell 18 in which the ground voltage GND is applied to the cathode 15 and, at the same time, a positive current is applied to the anode 12, thereby radiating the organic EL cell 18.
Such an EL display device applies a scanning pulse lowered by a threshold voltage in response to a data pulse to the scan lines SL. If a magnitude of the scanning pulse is lower than that of the data pulse by the threshold voltage of the EL cell 18, the panel generates a cross talk phenomenon. Thus, a voltage of the scanning pulse fails to have a low value. Further, since the scanning pulse is sequentially applied to the 1st to nth scan lines SL1 to SLn, a waveform shape of the scanning pulse must be constantly kept. In other words, in order to prevent a cross talk while constantly keeping a shape of the scanning pulse, a current magnitude of the scanning pulse must keep a magnitude similar to capacitances of the first and second switches T1 and T2 positioned at the scan driver 22.
In this case, since a time when a reverse bias is applied by the scan high voltage is longer than a time when a data voltage is applied, a line defect is caused by the scan high voltage and the current for a relatively long time. This is because the scan high voltage is relatively higher and hence a magnitude of the scan current is larger in proportion to it. In other words, when the panel has badness, a partially high stress is applied to heighten a probability of the line defect.
In a driving apparatus of the conventional EL display device, since the data driver 24 allows a current to be applied to the organic EL cell 18 in a data enable (DE) interval having a high state as shown in FIG. 5, the scan driver 22 does not need a relatively large current. On the other hand, since the data driver 24 supplies a low voltage or a ground voltage to the data lines DL in a data enable (DE) interval having a low state and the scan driver 22 must select the next scan line at the current scan line, the scan driver 22 needs a relatively large current. For instance, when it is intended to select the 2nd scan line SL2 at the 1st scan line SL1, a first scanning pulse applied to the 1st scan line SL1 is changed from a low state into a high state while a second scanning pulse applied to the 2nd scan line SL2 is changed from a high state into a low state. When the first scanning pulse is changed from a low state into a high state, the data enable signal DE also is changed from a high state into a low state. Thus, since a current from the data driver 24 has been applied to the organic EL cell 18, a relatively large amount of electrons are accumulated onto the cathode of the organic EL cell 18. Because these accumulated electrons must be eliminated, a relatively large current is required when the scanning pulse is changed from a low state into a high state. However, there is raised a problem in that a scan voltage and a current applied to the scan lines SL are always constantly supplied in the prior art.